I suspect that 1080p HEVC (higher bit rates) will not have an issue under tvOS11 on the ATV4. While FFMpeg does use neon, it is not the most optimal implementation.

I would expect that Apple's sw implementation of HEVC would perform much better. Plus under videotoolbox, frames would be decoded direct to ioSurfaces and this would skip one copy of the frame plus upload to GPU as a gles texture. This step saves some CPU/neon time. Seeing the positive results of other Apps that have taken the tvOS11 HEVC step, this is a very safe bet.

But 4k HEVC would still be difficult under the ATV4's A8. This is where the new ATV5 would shine. I suspect that Apple 'could' do hw decode on ATV4 but since it can only display 1080p, there's not much reason for it plus they need something to promote the move to ATV5s

Apple changed the spec of the ATV (current one), see also other thread:

After getting tvOS 11 I have HDMI-CEC issues. If I airplay from an iOS device it works great and my AVR switches inputs to the Apple TV. But if i reach for the ATV remote and hit, say menu, i see the light on the ATV but my inputs won't switch over. I have to switch inputs manually. It used to switch over automagically with any ATV command.